You can install this module either via composer or manually. Via composer:
composer require i-lateral/silverstripe-searchable
Alternativley install this module either by downloading and adding it to:
[silverstripe-root]/searchable
Finally run (either via the browser or command line):
dev/build flush=1
Running a dev build should import all search data into the search table (which is needed to perform accurate searches). If this doesn't happen (or you need to import data manually) there is a buid task that you can run:
sake dev/tasks/ImportSearchDataTask
Once installed, you will need to ensure you add the search form to your templates. This should be possible on any Controller using:
$SearchForm